Ionizing radiation (IR) is a well-established human carcinogen and exposure to IR is associated with the induction of hematological malignancies (HMs) and solid cancer. The prevalence and incidence of IR-induced HMs including therapy-related myelodysplastic syndrome and acute myelogenous leukemia (tMDS/AML) are rising because the survival of cancer patients treated with radiotherapy and/or chemotherapy improves and more patients receive CT scans for medical diagnosis particularly in children. Currently, tMDS/AML accounts for about 15% to 20% of all cases of MDS and AML and represents the most serious long-term complications for the patients with Hodgkin and non-Hodgkin lymphoma and several other cancers. Unfortunately, neither have the mechanisms by which IR induces HMs been elucidated nor has a strategy been developed to effectively prevent the induction of HMs by IR. These gaps will be addressed in this application. Specifically, we plan to test an original hypothesis that restoration of hematopoietic stem cell (HSC) fitness after IR via selective depletion of senescent HSCs with a senolytic drug that can selectively kill senescent cells (SCs) has the potential to be developed as a novel mechanism-based strategy to prevent IR-induced HMs. This is because new evidence suggests that induction of HMs by IR is in part attributable to a decrease in HSC fitness, which promotes clonal hematopoiesis and expansion of HSCs with preexisting and IR-induced oncogenic mutations to gain dominance and accumulate additional mutations for transformation. This hypothesis is also supported by our recent findings demonstrating that induction of HSC senescence was primarily responsible for the decrease of HSC fitness in mice after exposure to a sublethal dose of total body irradiation (TBI). Genetically or pharmacologically selective depletion of SCs and senescent HSCs rejuvenated the prematurely senescent HSCs induced by TBI and normally aged HSCs in old mice probably in part by stimulating the expansion of normal HSCs. In this application, three specific aims will be pursued to test our hypothesis using a mouse model: 1) to quantitatively and qualitatively determine the pool of normal HSCs preserved after exposure to a sublethal dose of TBI; 2) to determine whether genetic or pharmacological depletion of senescent HSCs after TBI can stimulate the expansion of normal HSCs, reduce IR-induced genomic instability, and suppress the expansion of mutated HSCs; and 3) to determine whether genetic or pharmacological depletion of senescent HSCs after TBI prevents the development of HMs. Our proposed studies will lead to a paradigm shift in prevention of IR-induced HMs by identifying new targets (e.g. SCs) and novel agents (e.g. senolytic drugs) for chemoprevention. Moreover, HSC senescence also occurs after chemotherapy and with age. Selective depletion of senescent HSCs with a senolytic drug may have broad applications for reduction of chemotherapy-induced secondary HMs in cancer patients, as well as de novo HMs in aged individuals, by improving the fitness of HSCs.